1. Field of the Invention
This invention relates to an objective lens for a disk regenerative optical system, and particularly to an objective lens directly driven for tracking.
2. Description of the Prior Art
Where a semiconductor laser is used as a light source, an objective lens used for regeneration of a videodisk or digital audiodisk must have a large aperture of about NA 0.45-0.5 and on-the-axis aberration need be held within a diffraction limit.
In these regenerative optical systems, where the objective lens is directly driven for tracking, an optical axis of the objective lens is always maintained vertically to the disk surface and moved parallel to the disk surface. Therefore, in principle, it is not necessary to correct off-axis aberration. Of course, it would be necessary to correct the sine sondition to some extent to minimize deterioration of performance due to error in mounting a light source, an objective lens, etc.
In order to directly drive the objective lens for tracking, the objective lens has to be disigned as small and as light as possible, and woring distance thereof has to be made sufficiently lengthy so that the disk may not contact therewith even if surface deviation of the disk should occur during rotation.
In order to achieve such an object, lens group has been put to practical use wherein it comprises 3 or 4 spherical glass lenses, the focal length is 4 to 5 mm, and working distance is about 2 mm. However, this lens group requires many lenses and it contains much difficulty so as to satisfy the required diffraction limit, performance when it is manufactured. Furthermore it is difficult to lower the manufacturing cost.
In the existing circumstances, therefore, the price of the objective lens among the prices of regenerative optical systems of videodisk and digital audiodisks accounts for great percent. In order to spread regenerative apparatus of videodisk and digital audiodisk by cost-down, considerable cost-down of the objective lens is required.
Recently, it becomes possible to make a refraction surface aspherical to thereby satisfy the diffraction limit performance despite a single lens, and an attempt has been made to use this for the regenerative objective lens of videodisk and digital audiodisk. In such a single lens, if a spherical surface is used for one refraction surface, it will be very advantageous in working and measurement.
Such an objective lens has been known from the first embodiment in Japanese Patent Laid-Open Nos. 17409/83, 68711/83 and the like. However, in these well-known examples, tracking is carried out by swinging a galvanometer, and correction of aberration required is different and working distance is short. In the latter patent, correction of sine condition is insufficient.